The method of constructing wells using casing as the drill string, where the bottom hole drilling assembly is deployed through the casing, does not permit incorporating devices such as a cement float shoe directly into the casing string in the conventional manner. Furthermore, the casing cannot be provided with an internally upset interval, on which to land a device introduced after drilling, as this would restrict the casing internal diameter preventing deployment of the bottom hole drilling assembly. In Canadian patent application CA 2,311,160, Vert and Angman disclose a cement float that can be positioned downhole in a casing string provided with a suitable profile nipple.
The function of a typical installed cement float requires it to act as a check valve allowing flow down a casing string suspended in a borehole but preventing backflow, sealing the casing bore from differential bottom pressure. This pressure differential exists during well cementing processes after wet cement is placed in the casing and displaced into the borehole-casing annulus by a lighter fluid. It is created by the difference in hydrostatic head between the cement and a lighter displacing fluid, commonly water, and in turn induces an axial load that must be reacted into the casing. This axial load increases with the differential pressure and the sealed area. Thus, the required structural capacity of such devices is greater for larger diameter casing and deeper wells.
Other devices must also be anchored downhole such a packers and other valves. These devices can also require anchoring arrangements that operate in pressure differentials.